dbi-link - dbi-link: - Added new function

Log Message:
-----------
- Added new function dbi_link.prepend_to_search_path(TEXT) which does
  what it sounds like, only it has the nice feature of being
  idempotent, which is to say that subsequent calls don't do more than
  the first one did.

- Modified the rest of dbi_link to use it along with the examples.

Modified Files:
--------------
    dbi-link:
        dbi_link.sql (r1.17 -> r1.18)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/dbi_link.sql.diff?r1=1.17&r2=1.18)
    dbi-link/examples/cache:
        cache.sql (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/cache/cache.sql.diff?r1=1.1&r2=1.2)
    dbi-link/examples/excel:
        excel.sql (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/excel/excel.sql.diff?r1=1.1&r2=1.2)
    dbi-link/examples/mssql:
        mssql.sql (r1.2 -> r1.3)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/mssql/mssql.sql.diff?r1=1.2&r2=1.3)
    dbi-link/examples/mysql:
        mysql.sql (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/mysql/mysql.sql.diff?r1=1.1&r2=1.2)
    dbi-link/examples/oracle:
        oracle.sql (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/oracle/oracle.sql.diff?r1=1.1&r2=1.2)
    dbi-link/examples/postgresql:
        postgresql.sql (r1.1 -> r1.2)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/postgresql/postgresql.sql.diff?r1=1.1&r2=1.2)